1. Field of the Invention
The present invention relates to a developing device utilizing electrophotographic developer, and more particularly to a developing device in which a thin uniform layer of a one-component developer, particularly an insulating non-magnetic one-component developer is formed on developer support means and is brought into facing relationship to a latent image bearing member for developing the latent image formed thereon.
2. Description of the Prior Art
There are already known or proposed various developing devices utilizing one-component developer. Among these is the so-called toner transition development device in which one-component developer is applied on the developer support means as a thin uniform layer which is then brought into facing relationship to an electrostatic latent image bearing surface with a small clearance therebetween. The developer is then caused to fly from the developer support means to the image bearing surface by the electrostatic attractive force to achieve image development, as disclosed in the Japanese Patent Publication Sho41-9475 and the U.S. Pat. No. 2,839,400. Such development provides a satisfactory image completely free from background fog, as the non-image area does not attract the developer and moreover is not even brought into contact with the developer. Also the absence of carrier particles in the one-component developer avoids the troubles resulting from the variation in the mixing ratio of developer or from the deterioration of the carrier particles.
Apart from the aforementioned development method and device, the present applicant proposed novel developing methods as disclosed in the U.S. patent application Ser. Nos. 938,101 and 938,494 and in Ser. Nos. 58,434 and 58,435, now U.S. Pat. No. 4,292,387, issued Sept. 9, 1981.
The former method utilizes one-component magnetic developer, non-magnetic developer support means and magnetic-field generating means arranged in this order to form a thin uniform layer of the developer support means by the magnetic force caused by said magnetic-field generating means. The developer layer is brought into facing relation to an electrostatic latent image bearing surface with a small clearance therebetween, whereby the developer present corresponding to an image area is extended by the electrostatic attractive force of said latent image to achieve image development. This method also provides an image completely free from background fog as the development is effected without the contact of developer with the non-image area.
The latter method utilizes one-component magnetic developer, non-magnetic developer support means and magnetic-field generating means arranged in this order to form a thin uniform layer of the developer on the developer support means by the magnetic force caused by said magnetic-field generating means. The developer layer is brought into facing relation to an electrostatic latent image bearing surface with a small clearance therebetween, wherein the image development is carried out by applying an AC developing bias field across said small clearance and creating a time-dependent change in the clearance between said latent image bearing surface and the developer support means. In such method the developer also reaches the non-image area of the electrostatic latent image in the earlier state of the development to achieve image development in the halftone image area, but remains in contact only with the image area in the latter state. This method thus provides a development without background fog and with improved halftone reproduction in comparison with the above-mentioned developing method.
As explained in the foregoing, the developing methods utilizing a thin layer of developer maintained in facing relation to the latent image bearing surface are far superior, with respect to the developing performance, image reproducibility and service life of developer, to other conventionally known developing methods.
Preferred in such new developing methods is the use of toner of high electric resistance or insulating toner, in consideration of the ease of transfer onto plain paper.
One-component magnetic developer, due however to the magnetic particles contained in the developer particles, is more expensive than the non-magnetic developer and is difficult to produce in satisfactorily pure colors. Besides the developing device for magnetic developer is not only heavy but also expensive due to the use of magnets behind the developer support means. For these reasons the developing method and device utilizing one-component non-magnetic developer is desirable.
In fact the latent image development with one-component non-magnetic developer has long been known in the filed of electrophotography, but such conventionally known method has been not entirely satisfactory in being characterized by poor service life of the developer, leading to the gradual decrease in developed density and in the difficulty of uniform and stable developer application on the developer support means. Also such method has been associated with a phenomenon of a difference in the thickness of developer layer on the developer support means between an area in which the developer is taken away by the preceding development and an area not subjected to such preceding removal of developer, said difference being reproduced as a difference in density in the image thus obtained.
There is also proposed a method of applying developer onto the developer support means having surface irregularities, but the above-mentioned phenomenon is still associated with such method. In order to prevent such phenomenon there is proposed a method of removing the remaining developer from the developer support means after the development and uniformly applying the developer anew on said support means. Such removal of developer is also effective in preventing the surface of said support means from being covered by minute particles of the developer or other substances, which otherwise undesirably affects the frictional charging between the developer and said support means. In such method, however, the support means with surface irregularities is not suitable as such irregularities will hinder the removal of developer. On the other hand the support means with a smooth surface will inevitably limit the thickness of developer layer uniformly applicable thereon, thus leading to a low image density. The abovementioned method has thus not been entirely satisfactory for uniform and stable developer application, for stable and uniform developer charging and for stability in obtainable image density.